Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
  • C25B11/04—Electrodes; Manufacture thereof not otherwise provided for characterised by the material
  • C25B11/051—Electrodes formed of electrocatalysts on a substrate or carrier
  • C25B11/073—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material
  • C25B11/091—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material consisting of at least one catalytic element and at least one catalytic compound; consisting of two or more catalytic elements or catalytic compounds
  • C25B11/093—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material consisting of at least one catalytic element and at least one catalytic compound; consisting of two or more catalytic elements or catalytic compounds at least one noble metal or noble metal oxide and at least one non-noble metal oxide
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
  • C25B11/04—Electrodes; Manufacture thereof not otherwise provided for characterised by the material

Definitions

• This invention relates to an improved type of coating intended for constituting the active surface of a metal electrode of use in the electrolysis of alkali metal halides, and, particularly, in the production of sodium chlorate from said electrolysis.
• an aqueous solution of an alkali metal halide is electrolyzed to produce chlorine at the anode and an alkali hydroxide and hydrogen at the cathode.
• the products of electrolysis are maintained separate.
• sodium chlorate the chlorine and alkali hydroxide are allowed to mix at almost neutral pH and the sodium chlorate is formed via disproportionation of the sodium hypochlorite formed in the above mixing.
• U.S. Pat. No. 3,849,282--Deguldre et al. describes a coating for metal electrodes, which coating comprises a compound ABO 4 having a rutile-type structure, where A is an element in the trivalent state selected from the group rhodium, aluminum, gallium, lanthanum and the rare earths, while B is an element in the pentavalent state selected from the group antimony, niobium and tantalum, the compound ABO 4 being associated with an oxide of the type MO 2 where M is ruthenium and/or iridium.
• the electrodes described therein may be used in various electrochemical processes such as cathodic protection, desalination or purification of water, electrolysis of water or hydrochloric acid, production of current in a fuel cell, reduction or oxidation of organic compounds for the electrolytic manufacture of per salts, and as anodes in the electrolysis of aqueous solutions of alkali metal halides, particularly sodium chloride, in diaphragm cells, mercury cells, membrane cells and chlorate production cells, where they catalyze the discharge of chloride ions.
• alkali metal halides particularly sodium chloride
• the electrodes described therein are stated to adhere to their metal support and are stated to be resistant to electrochemical attack.
• U.S. Pat. No. 3,718,551--Martinsons describes an electroconductive coating for metal electrodes, which coating comprises a mixture of amorphous titanium dioxide and a member of the group consisting of ruthenium and ruthenium dioxide.
• the electrodes described therein are characterized by having a low oxygen and chlorine overvoltage, resistance to corrosion and decomposition for coatings containing less than 60% by weight of titanium (as oxide) based on the total metal content of the coatings.
• a further source of oxygen in chlorate-production cells can arise due to catalytic decomposition of the intermediate sodium hypochlorite by metallic contaminants.
• the platinum metal oxides used as electrocatalytic coatings for chloride oxidation are also excellent catalysts for hypochlorite decomposition. It is important, therefore, not only for long uniform performance life of the anode coating but also to minimize catalytic decomposition of the sodium hypochlorite that strongly adhering electrocatalytic coatings should be employed on electrodes for the electrolysis of halide solutions.
• electrocatalytic coatings produced solely from platinum group metal compounds can, depending upon the platinum metal used, be expensive. It is desirable, therefore, that provided the operating characteristics of low oxygen evolution, low voltage, low wear rate are satisfied, the proportion of platinum group metal in the coating should be as low as possible.
• the invention provides a metallic electrode for electrochemical processes comprising a metal support and on at least a portion of said support a conductive coating consisting essentially of a mixed oxide compound of (i) a compound of the general formula ABO 4 having a structure of the rutile-type, where A is an element in the trivalent state selected from the group consisting of Al, Rh, and Cr, and B is an element in the pentavalent state selected from the group consisting of Sb and Ta, (ii) RuO 2 and (iii) TiO 2 ; wherein the mole fraction of ABO 4 is between 0.01 and 0.42, the mole fraction of RuO 2 is between 0.03 and 0.42 and the mole fraction of TiO 2 is between 0.55 and 0.96.
• ABO 4 a compound of the general formula ABO 4 having a structure of the rutile-type, where A is an element in the trivalent state selected from the group consisting of Al, Rh, and Cr, and B is an element in the pentavalent state selected from the group consisting of
• the electrodes have low precious metal content and provide low wear rates and improved current efficiency-anodic overvoltage performance. They are used in the electrolysis of chloride containing liquors in the production of, for example, chlorine, and, particularly chlorate.
• the conductive coating of use in the present invention on a metal support at least superficially made of titanium or a metal of the titanium group.
• titanium is clad on a core of a more conductive metal such as copper, aluminum, iron, or alloys of these metals.
• the coating of use in the present invention consists essentially of the compounds as defined hereinabove in the relative amounts defined; yet more preferably, the coating consists of those compounds as defined.
• the compounds ABO 4 , RuO 2 and TiO 2 must be present together in the coating in the relative amounts defined whether or not a further constituent is present in the coating.
• This Example illustrates the preparation and properties of an electrode having a coating of the formula:
• a solution x was prepared by dissolving 0.54 gms of AlCl 3 and 1.21 gms of SbCl 5 in 40 mls of n-butanol and a solution y was prepared by dissolving 2.0 gms of finely ground RuCl 3 .xH 2 O(40.89% Ru) in 40 mls of n-butanol.
• Solutions x and y were brought together with 13.1 mls (CH 3 (CH 2 ) 3 O) 4 Ti and mixed well. This solution was applied in six layers onto plates of titanium which had previously been hot-degreased in trichloromethylene, vacu-blasted, and then etched for seven hours at 80° C. in 10% oxalic acid solution. After each application of the coating mixture the plates were dried with infra-red lamps and then heated in air for fifteen minutes at 450° C. After the sixth coating application the titanium plates, now fully coated, were heated for 1 hour at 450° C. The amount of material thus deposited was about 8 g/m 2 .
• the coating which had a mole fraction of AlSbO 4 of 0.08, RuO 2 of 0.17 and TiO 2 of 0.75 showed excellent adherence to the titanium substrate, as was shown by stripping tests with adhesive tape applied by pressure, both before and after operation in electrolytic cells for the production of sodium chlorate.
• the titanium plates thus coated were submitted to four further types of evaluation.
• the first evaluation relates to the electrode performance with regard to oxygen formation when used in a cell producing sodium chlorate under commercial conditions.
• the second evaluation relates to the anodic voltage when the electrode is used under typical conditions of commercial sodium chlorate production.
• the third evaluation relates to the performance of the coating under accelerated wear tests under conditions where the final anodic product is sodium chlorate but the production conditions are very much more aggressive than those encountered in commercial practice.
• the fourth evaluation relates to the performance of the coating under accelerated wear conditions where the anodic product is chlorine but the production conditions are very much more aggressive than those encountered in commercial practice.
• the first test was performed with an electrolyte at 80° C. containing 500 g/l NaClO 3 , 110 g/l NaCl and 5 g/l Na 2 Cr 2 O 7 .
• the electrolyte was circulated past the coated titanium anode produced above at a fixed rate in terms of liters/Amp-hour and the oxygen measured in the cell off-gases over a range of current densities between 1 and 3 kA/m 2 .
• the second test was performed with the same apparatus as for the first test but with a Luggin capillary probe used to measure the anodic voltage at various current densities before and after prolonged operation.
• a Luggin capillary probe used to measure the anodic voltage at various current densities before and after prolonged operation.
• the third test was performed using an electrolyte containing 500 g/l of NaClO 3 and only 20 g/l of NaCl with 5 g/l Na 2 Cr 2 O 7 .
• the electrodes were operated in a chlorate production cell at 80° C. and 5 kA/m 2 . (See, for example, An Accelerated Method of Testing The Durability of Ruthenium Oxide Anodes for the Electrochemical Process of Producing Sodium Chlorate, L. M. Elina, V. M. Gitneva and V. I. Bystrov., Elektrokimya, Vol. II, No. 8, pp 1279-1282, August 1975).
• the fourth test was performed using an electrolyte containing 1.85M HClO 4 and 0.25M NaCl.
• the electrodes were operated in a chlorine production cell at 30° C. and at constant cell voltage using a potentiostat. The current under constant voltage was recorded until it changed significantly which indicated the time-to-failure of the test electrode.
• Electrochemical Behaviour of the Oxide-Coated Metal Anodes See, for example, Electrochemical Behaviour of the Oxide-Coated Metal Anodes, F. Hine, M. Yasuda, T. Noda, T. Yoshida and J. Okuda., J. Electrochem Soc., September 1979, pp 1439-1445).
• the oxygen content of the gases exiting the chlorate production cell in the first test was 1.5% at 2kA/m 2 at 80° C. for the electrode prepared in the above example.
• the anode voltage was measured to be 1.14 volts vs. S.C.E. also at 2kA/m 2 and 80° C.
• the sample electrode was rechecked after running for 103 days under the same operating conditions as in the first test and the result showed no change in anodic voltage.
• the cell voltage started to rise after nine days of operation under accelerated wear testing conditions for chlorate production (an indication of time-to-failure), but the coating was still strongly adherent on the substrate.
• the resistivity of the coating increased significantly after two hours of operation under accelerated wear testing conditions for chlorine production.
• the AlSbO 4 RuO 2 coating was characterized by a high voltage and poor mechanical stability.
• the RuO 2 .TiO 2 coating demonstrated a much higher oxygen evolution and therefore lower efficiency and poorer overall performance.
• the coating, the subject of this invention demonstrated a superior overall electrochemical performance.
• accelerated testing of the mixed coating, the subject of this invention indicated a superior life to that of the RuO 2 TiO 2 admixture and in this respect it is noted that commercial coatings of this general composition usually contain more than 20% MF RuO 2 . It was also surprising that the AlSbO 4 RuO 2 coating demonstrated such poor stability in the light of the teachings of U.S. Pat. No. 3,849,282.
• This Example illustrates the preparation and properties of an electrode having a coating of the formula:
• a solution x was prepared by adding 0.53 gms AlCl 3 and 1.44 gms TaCl 5 to 40 mls of n-butanol.
• a solution y was prepared by dissolving 2.0 gms of finely ground RuCl 3 1-3H 2 O (40.2% Ru) in 40 mls of n-butanol.
• the oxygen content of the gases exiting the cell was 1.4% at 2kA/m 2 and 80° C.
• the anodic voltage under the same operating conditions was 1.14 volts vs. S.C.E.
• the accelerated wear test using the chlorate electrolyte with low chloride content, (third test) showed that the cell voltage started to rise after 14 days of operation.
• the resistivity of the coating increased significantly after 0.5 hours of operation under accelerated wear testing conditions for chloring production for the above electrode.
• This coating confirms the beneficially synergistic effect of the classes of components, the subject of this invention.
• This Example illustrates the preparation and properties of an electrode having a coating of the formula:
• a solution x was prepared by adding 1.16 gms CrBr 3 and 1.19 gms SbCl 5 to 40 mls of n-butanol.
• a solution y was prepared by dissolving 2 gms of finely ground RuCl 3 .1-3H 2 O (40.2% Ru) in 40 mls of n-butanol. Solutions x and y were then mixed well with 12.9 mls of tetrabutyl orthotitanate (CH 3 (CH 2 ) 3 O) 4 Ti). The mixture was coated (6x) to a cleaned and etched titanium plate using the same techique as for Example 1. The amount of material deposited was about 8 g/m 2 .
• the coating stability was excellent.
• the anode voltage and the oxygen content of the gases exiting the cell were 1.11 volts vs. S.C.E. and 2% respectively under the same operating conditions as in Example 2.
• This coating demonstrates a further improvement in voltage than hitherto found and surprisingly well below that expected from earlier teachings.
• This Example illustrates the preparation and properties of an electrode having a coating of the formula:
• a solution x was prepared by adding 0.975 gms of RhCl 3 .xH 2 O (42.68% Rh) and 1.1 gms of SbCl 5 to 40 mls of n-butanol.
• a solution y was prepared by dissolving 2 gms of finely ground RuCl 3 .xH 2 O (40.89 T Ru) in 40 mls of n-butanol. Solutions x and y were then mixed well with 13.1 mls of tetrabutyl orthotitanate. The mixture was coated (6 ⁇ ) to a cleaned and etched titanium plate using the same technique as for Example 1. The amount of material deposited was about 8 g/m 2 .
• the coating showed excellent coating stability, both before and after operation in electrolytic cells for the production of chlorate. Under the same operating conditions as in Example 2, the anodic voltage and the oxygen content of the gases exiting the cell were found to be 1.13 volts vs. S.C.E. and 1.33% respectively. The overvoltage of the coating increased significantly after 6.5 hours of operation under accelerated wear testing conditions for chlorine production.
• This coating again demonstrates a significantly better voltage-current efficiency performance than would have hitherto been expected and potentially shows a further technical advantage of coating the subject of this invention where A is Rh over the previously exemplified Al.
• This Example illustrates the surprisingly good voltage-current efficiency performance of coatings of the general formula aABO 4 bRuO 2 cTiO 2 in relation to coatings of the type aABO 4 bRuO 2 and bRuO 2 cTiO 2 .
• the coatings were prepared as generally described for Example 1 with appropriate concentrations of the species required for the desired coating formulation.
• This Example illustrates the preparation and properties of further electrodes according to the invention.
• a series of coated titanium sheets was made up using the same technique as for Example 1. However, for these plates, the relative amounts of solutions x, y and butyl titanate were varied to provide coatings with a range of AlSbO 4 RuO 2 TiO 2 contents.
• the anodic voltages and oxygen contents of the cell gases of the various coated sheets are shown in Tables 3 and 4. The wear rates of all these coatings both before and after operation, as measured by the tape test were excellent.
• anode demonstrate anodic voltages of typically 1.14 volts vs. S.C.E. and off-gas oxygen concentreations of 2 to 3% under the above operating conditions.
• the anode according to the invention with a molar fraction of AlSbO 4 of 0.08 and RuO 2 of 0.17 has a comparable anodic voltage which is surprising from the teaching of Martinsons and, for this low anodic voltage a surprisingly high efficiency from the teaching of Kotowski and Busse.
• RuO 2 content results in coatings with constant oxygen evolution and surprisingly low overvoltages for the low RuO 2 contents when compared to commercial RuO 2 TiO 2 coatings which contain RuO 2 at typically above 0.3 MF and ABO 4 RuO 2 coatings which contain RuO 2 at typically 0.5 MF.
• Example 1 illustrates the surprisingly good oxygen overpotentials to oxygen evolution relationship of the electrodes according to the invention.
• a coated titanium sheet was made up using the same technique as for Example 1.
• titanium sheets were made up using the technique generally described for Example 1 to give admixtures separately of RuO 2 TiO 2 and RhSbO 4 RuO 2 .
• This Example illustrates the surprisingly good oxygen overpotentials of the electrodes according to the invention as a function of operating temperature.
• Coated titanium sheets were made up using the same technique as for Example 1.
• titanium sheets were made up using the technique generally described for Example 1 to give a coating of the composition AlSbO 4 .2RuO 2 .
• the oxygen overpotential of these electrodes was measured as described in Example 7 over a range of temperatures. The results are given in Table 6.
• the electrodes, the subject of the invention show a reduced temperature effect on oxygen overpotential and in turn facilitate the opportunity for further process improvements in the ability for coatings, the subject of this invention, to operate satisfactory electrolysis applications at temperatures higher than that traditionally considered inoperable.

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• Electrodes For Compound Or Non-Metal Manufacture (AREA)
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• Electrolytic Production Of Non-Metals, Compounds, Apparatuses Therefor (AREA)

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